A helmet configured as described above is conventionally known, as disclosed in US 2009/0100576 A1. In the helmet (to be referred to as “the conventional helmet” hereinafter) disclosed in US 2009/0100576 A1, each of the left and right chin guard support mechanisms includes a fixed-side unit fixed to the left side surface or right side surface of the main cap body, and a pivotal-side unit fixed to a region including the left or right end of the chin guard and a vicinity thereof. The fixed-side unit is provided with a first longitudinal guide hole that extends forward obliquely above, and a second longitudinal guide hole that is arranged under the first longitudinal guide hole and extends forward obliquely below. The longitudinal direction of the first longitudinal guide hole is substantially perpendicular to that of the second longitudinal guide hole. The first and second longitudinal guide holes extend substantially linearly. The pivotal-side unit is provided with a first connection portion that is inserted into the first longitudinal guide hole and guided by the first longitudinal guide hole, and a second connection portion that is inserted into the second longitudinal guide hole and guided by the second longitudinal guide hole.
In the conventional helmet configured as described above, when the chin guard moves from the lowermost position to the uppermost position, the second connection portion moves from the upper end side of the second longitudinal guide hole to its lower end side. At the same time, the first connection portion moves from the lower end side of the first longitudinal guide hole to its upper end side and then from the upper end side to the lower end side. Hence, during the movement from the lowermost position to the midpoint of rise, the chin guard slightly moves forward, too. In addition, during the rise from the midpoint of rise to the uppermost position, the chin guard slightly moves backward, too. When lowering from the uppermost position to the lowermost position contrary to the rise, the chin guard slightly moves forward and then slightly moves backward, as in the rise.
However, in the left or right chin guard support mechanism of the conventional helmet, the fixed-side unit having the first and second longitudinal guide holes, and the pivotal-side unit including the first and second connection portions need to be provided between the left side surface or the right side surface of the main cap body and a region including the left or right ends of the chin guard and a vicinity thereof. For this reason, the structure of each chin guard support mechanism is relatively complex, and it may be difficult to smoothly operate the chin guard support mechanisms. In addition, a space to provide the fixed-side unit and the pivotal-side unit is necessary on the left or right side surface of the main cap body and the region including the left or right ends of the chin guard and the vicinity thereof. For this reason, the structure in a region including the end of the chin guard including the left or right chin guard support mechanism and a vicinity thereof may be bulky, and the widthwise size of the helmet including the left and right chin guard support mechanisms may be large.